Internal combustion engines burn fuel to generate a drive torque, which may power numerous types of devices including handheld power tools, automobiles, and many other machines. One type of internal combustion engine is a reciprocating engine or, as more commonly termed, a piston engine.
A typical piston engine includes an engine block, one or more pistons, a crankshaft, a valve assembly, and a fuel delivery system. The engine block defines one or more cylindrical cavities, which are referred to simply as cylinders. Some piston engines include only one cylinder, while other piston engines include eight or more cylinders. For simplicity, this background describes a piston engine having only one cylinder. The piston is a generally cylindrical unit that is positioned for movement within the cylinder. A tie rod connects a bottom side of the piston to the crankshaft. A portion of the cylinder located above a top side of the piston is referred to as a combustion chamber. The valve assembly is connected to the engine block for fluid communication with the cylinder and, in particular, with the combustion chamber. The fuel delivery system supplies the combustion chamber, either directly or indirectly, with a supply of fuel. Piston engines may include additional components, systems, and subsystems as known to those of ordinary skill in the art.
In operation, the exemplary piston engine generates a drive torque by periodically burning a fuel within the combustion chamber. Exemplary fuels include, but are not limited to, gasoline, diesel, ethanol, and alcohol. Burning the fuel within the combustion chamber generates a force, which causes the piston to move within the cylinder. Movement of the piston causes the crankshaft to rotate, thereby generating the rotational drive torque of the engine. The exhaust by-products of the burnt fuel are expelled from the combustion chamber through the valve assembly.
Most piston engines generate the drive torque with either a two-stroke operation cycle or a four-stroke operation cycle. A “stroke” of the piston refers to the piston moving from near one end of the cylinder to near the opposite end of the cylinder. For example, in an engine having a vertically oriented cylinder, a stroke occurs when the piston slides from near the top of the cylinder to near the bottom of the cylinder. A stroke also occurs when the piston slides from near the bottom of the cylinder to near the top of the cylinder.
The four-stroke operation cycle sequentially performs four engine operations over the course of four strokes of the piston. The strokes associated with these engine operations are commonly termed the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. During the intake stroke, the piston moves away from the valve assembly and draws fresh air into the combustion chamber. Also, during the intake stroke fuel enters the combustion chamber. Next, during the compression stroke, the piston moves toward the valve assembly and compresses the fresh air and fuel mixture within the combustion chamber. During the power stroke, the fuel within the combustion chamber is burned to generate rapidly expanding gases, which are referred to as exhaust by-products. The exhaust by-products force the piston away from the valve assembly. Next, during the exhaust stroke, the piston moves toward the value assembly, and the valve assembly evacuates the exhaust by-products from the combustion chamber. A piston engine operating with the four-stroke operation cycle (referred to as a four-stroke engine) sequentially repeats these strokes very rapidly to generate the crankshaft drive torque.
Piston engines operating with a two-stroke operation cycle perform the four engine operations described above over the course of two strokes of the piston. The strokes associated with these engine operations are commonly called a combination intake and compression stroke and a combination power and exhaust stroke. During the combination intake and compression stroke, the piston moves toward the valve assembly to draw fresh air into the combustion chamber and to compress the fresh air and fuel within the combustion chamber. Next, during the power and exhaust stroke, the fuel within the combustion chamber is burned, which generates exhaust by-products and causes the piston to move away from the valve assembly. As the piston moves away from the valve assembly the exhaust by-products are evacuated from the combustion chamber. A piston engine operating with the two-stroke operation cycle (referred to as a two-stroke engine) sequentially repeats these strokes very rapidly to generate the crankshaft drive torque.
Both two-stroke and four-stroke engines may exhibit a process referred to as short circuiting. It is typically desirable for two-stroke and four-stroke engines to evacuate completely the exhaust by-products from the combustion chamber without allowing any portion of the fresh air and/or unburned fuel to be evacuated from the combustion chamber with the exhaust by-products. During short circuiting, however, a portion of the fuel and/or fresh air is evacuated from the cylinder with the exhaust by-products. The fuel that is evacuated from the combustion chamber without being burned does not contribute to the generation of the drive torque. Accordingly, short circuiting reduces the fuel efficiency of an internal combustion piston engine. Therefore, further developments in the area of internal combustion piston engines are desirable.